Fibril cellulose refers to isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Fibril cellulose, which is also known as nanofibrillar cellulose (NFC) and by other related names, is based on a natural polymer that is abundant in nature. Fibril cellulose has many potential uses for example based on its capability of forming viscous gel in water (hydrogel).
Fibril cellulose production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers. The concentration of fibril cellulose in dispersions is typically very low, usually around 1-5%. After the grinding process, the obtained fibril cellulose material is a dilute viscoelastic hydrogel. The material itself is usable as such in many applications, but logistic costs are too high to transport the material from the production site. In some applications, the high water content is not acceptable, i.e. the formulations do not tolerate large amounts of water.
Thus, there is an evident need for increasing the concentration of the final product so that the transport costs would be decreased and the NFC could be used in the final destination at a suitable concentration desired by the end user by simply redispersing the fibril cellulose in water.
Strong water retention is typical for fibril cellulose since water is bound to the fibrils through numerous hydrogen bonds. Conventional separation techniques for lowering the water content, such as filtration or evaporation are not feasible with fibril cellulose hydrogels.
The fundamental problem in mechanical water removal is the ability of fibril cellulose hydrogel to form a very dense and impermeable nanoscale membrane around itself, for example during filtration. The formed shell prevents diffusion of water from the gel structure, which leads to very slow concentration rates. The same applies to vacuum evaporation where the skin formation blocks the evaporation of water.
Another problem in drying of fibril cellulose is the non-redispersibility of the dried material. During the water removal, the fibril-water bonds are replaced with fibril-fibril interactions and the fibrils are permanently aggregated. This can be prevented with the use of certain additives during the drying stage, such as CMC, or by chemical modification of the microfibril surface, e.g. oxidation or carboxymethylation. With those methods fibril cellulose can be re-activated after complete drying.
In the literature, the use of organic solvents in separation of fibril cellulose from water has been described. The proposed processes have been based on precipitation of dilute fibril cellulose dispersion into a non-solvent, such as isopropanol. Precipitation is typically carried out from dilute solutions with high speed mixing.
Methods for dewatering fibril cellulose are known for example from international publication WO-0166600 and European patent EP-0859011.